Daily or circadian rhythms controlled by endogenous clocks and synchronized to the solar cycle are characteristic of all organisms. The primary mammalian circadian clock, in the suprachiasmatic nuclei (SCN), survives in vitro where its phase can be monitored through its 24 hr rhythm of neuronal activity. Changes in the phase of this rhythm reliably reflect phase changes in the underlying clock. In the SCN, phase is modulated by afferents from the retina, intergeniculate leaflet of the lateral geniculate nucleus, and raphe nuclei. The primary neurotransmitters for these inputs are an excitatory amino acid (possibly glutamate), neuropeptide Y (NPY) and gamma-aminobutyric acid (GABA), and serotonin (5-HT), respectively. These afferents synapse primarily onto vasoactive intestinal polypeptide (VIP)-containing SCN cells, and possibly converge onto the exact same cells. Thus, in addition to individually modulating clock phase, it is likely that these afferent systems influence each others effects on SCN clock phase. While previous studies indicate that each of these afferent neurotransmitters can phase-shift the SCN pacemaker, both in vivo and in vitro, when applied individually, there is scant information concerning possible interactions between these inputs, and even less is known about the mechanisms through which these interactions might occur. This proposal represents the first phase of a broad investigation into afferent modulation of SCN clock phase. These experiments investigate 1. How 5-HT, NPY, GABA, glutamate, and optic chiasm stimulation affect the SCN in vitro when applied individually, determining in particular A) if and when they phase-shift the clock, and B) what their acute effects on SCN neuronal activity are when applied during the day and night; and 2. How 5-HT, NPY, glutamate, GABA, and optic chiasm stimulation modulate each other's effects in the SCN, investigating whether applying these stimuli together affects A) each other's pattern of phase-shifting, and B) their acute effects on the firing rates of SCN cells. This will be the first systematic investigation of interactions between SCN afferents in vitro, and as such it should provide critical information concerning the basic mechanisms underlying the mammalian circadian system. In addition, the increased understanding of how the SCN circadian pacemaker can be manipulated by external stimuli should produce rapid advances in our ability to alleviate problems that have been linked to circadian rhythm disorders, including sleeplessness, narcolepsy, and manic depression, as well as the medical and performance problems associated with jet lag and shift work schedules.